An ecosystem is a functional unit of nature where living organisms (biotic components) interact with non-living components (abiotic components) such as air, water, soil, light, and temperature.

In simple words,
👉 Ecosystem = Living organisms + Non-living environment + Interaction

The chapter “Ecosystem” in Class 12 Biology explains:

• Structure of ecosystem
• Energy flow
• Productivity
• Decomposition
• Nutrient cycling
• Ecological pyramids
• Food chain and food web

This chapter is very important for CBSE board exams, NEET, and competitive exams.

Class 12 Biology Ecosystem Notes PDF | NCERT Based Important Questions 2026

🌍 Types of Ecosystem

Ecosystems are broadly divided into two main types:

1️⃣ Natural Ecosystem

These occur naturally without human interference.

(a) Terrestrial Ecosystem

• Forest ecosystem
• Grassland ecosystem
• Desert ecosystem

(b) Aquatic Ecosystem

• Freshwater ecosystem (pond, lake, river)
• Marine ecosystem (sea, ocean)

2️⃣ Artificial Ecosystem

These are man-made ecosystems created and maintained by humans. Examples:

• Crop fields
• Aquarium
• Garden

🧬 Structure of an Ecosystem

The structure of an ecosystem consists of two main components:

1️⃣ Abiotic Components (Non-Living)

These include physical and chemical factors that affect living organisms.

Major Abiotic Factors:

• Light – main source of energy
• Temperature – affects metabolism
• Water – essential for life
• Soil – provides minerals
• Inorganic substances – carbon, nitrogen, phosphorus
• Organic substances – carbohydrates, proteins, lipids

2️⃣ Biotic Components (Living)

Biotic components are divided into three categories:

🌱 (a) Producers (Autotrophs)

• Green plants
• Algae
• Photosynthetic bacteria

🔹 They prepare their own food by photosynthesis
🔹 They form the base of the food chain

🐄 (b) Consumers (Heterotrophs)

They depend on producers for food.

Types of Consumers:

1. Primary consumers – Herbivores (deer, rabbit)
2. Secondary consumers – Carnivores (frog, lizard)
3. Tertiary consumers – Top carnivores (lion, tiger)

🍄 (c) Decomposers

• Bacteria
• Fungi

🔹 They break down dead plants and animals
🔹 They recycle nutrients back to the ecosystem

⚡ Energy Flow in an Ecosystem

Energy flow in an ecosystem is unidirectional.

☀ Sun → 🌱 Producers → 🐄 Consumers → 🍄 Decomposers

Important Points:

• Only 2–10% of energy is transferred from one trophic level to the next
• Rest energy is lost as heat (according to Second Law of Thermodynamics)
• Energy flow follows Lindeman’s 10% Law

🌾 Productivity in an Ecosystem

Productivity refers to the rate of biomass production.

Types of Productivity:

1️⃣ Primary Productivity

Rate at which producers synthesize food.

(a) Gross Primary Productivity (GPP)

• Total energy fixed by plants through photosynthesis

(b) Net Primary Productivity (NPP)

• Energy available to consumers

📌 NPP = GPP – Respiration (R)

2️⃣ Secondary Productivity

Rate of biomass production by consumers.

🍂 Decomposition in Ecosystem

Decomposition is the process of breaking down dead organic matter into simple substances.

Steps of Decomposition:

1. Fragmentation
   • Breaking of detritus into smaller particles
   • Done by earthworms
2. Leaching
   • Water dissolves nutrients and carries them into soil
3. Catabolism
   • Bacteria and fungi degrade organic matter
4. Humification
   • Formation of dark coloured humus
5. Mineralisation
   • Release of inorganic nutrients

♻ Nutrient Cycling (Biogeochemical Cycles)

Movement of nutrients from environment → organisms → back to environment.

🌬 Gaseous Cycle

• Carbon cycle
• Nitrogen cycle

Reservoir: Atmosphere

🪨 Sedimentary Cycle

• Phosphorus cycle
• Sulphur cycle

Reservoir: Earth’s crust

🌱 Carbon Cycle (Brief)

• CO₂ fixed by plants during photosynthesis
• Returned to atmosphere through respiration, decomposition, burning

🔺 Ecological Pyramids

Graphical representation of trophic levels.

Types of Ecological Pyramids:

1️⃣ Pyramid of Number

• Shows number of organisms
• Can be upright or inverted

2️⃣ Pyramid of Biomass

• Shows total biomass
• Upright in terrestrial ecosystem
• Inverted in aquatic ecosystem

3️⃣ Pyramid of Energy

• Always upright
• Most accurate pyramid

🍃 Food Chain

A linear sequence showing transfer of energy.

Types:

1. Grazing food chain
   Example: Grass → Deer → Lion
2. Detritus food chain
   Example: Dead leaves → Earthworm → Bird

🕸 Food Web

Interconnected food chains forming a network.

Importance:

• More stable than food chain
• Prevents extinction of species

🌍 Ecosystem Services

Benefits provided by ecosystems.

Major Ecosystem Services:

• Oxygen production
• Pollination
• Climate regulation
• Soil fertility
• Water purification

📌 Value of ecosystem services is very high and mostly free of cost.

📌 Important Points to Remember (Exam Focus)

✔ Energy flow is unidirectional
✔ Decomposers are essential for nutrient recycling
✔ Pyramid of energy is always upright
✔ Only 10% energy transfer between trophic levels
✔ NPP is available to consumers

📝 Conclusion–Class 12 Biology Ecosystem Notes PDF

The ecosystem maintains balance in nature by ensuring proper energy flow, nutrient cycling, and interaction among organisms. Understanding ecosystem is crucial for environmental conservation and sustainable development. This chapter builds a strong foundation for ecology in higher studies.

📌 Class 12 Biology Ecosystem Notes – Important Repeated Questions

🔹 Very Short Answer Questions (1 Mark)

1. What is an ecosystem?
2. Name the two main components of an ecosystem.
3. Define producers.
4. Who are decomposers?
5. What is detritus?
6. Name the law that explains energy transfer in ecosystem.
7. What is GPP?
8. What is NPP?
9. Which ecological pyramid is always upright?
10. Name one artificial ecosystem.

🔹 Short Answer Questions (2 Marks)

1. Differentiate between biotic and abiotic components of an ecosystem.
2. What is a food chain? Give one example.
3. Why are decomposers important in an ecosystem?
4. Define primary productivity.
5. What is secondary productivity?
6. What is the role of sunlight in ecosystem?
7. Why is energy flow unidirectional?
8. What is leaching in decomposition?
9. Name the steps of decomposition.
10. What is ecological pyramid?

🔹 Short Answer Questions (3 Marks)

1. Explain the structure of an ecosystem.
2. Describe the types of consumers with examples.
3. Write a short note on food web.
4. Explain grazing food chain and detritus food chain.
5. What is decomposition? Explain any three steps.
6. Explain pyramid of numbers with an example.
7. What is nutrient cycling? Name its types.
8. Why is pyramid of energy always upright?
9. Describe abiotic components of ecosystem.
10. Explain humification and mineralisation.

🔹 Long Answer Questions (5 Marks)

🔥 Most Repeated in Board Exams

1. Describe the energy flow in an ecosystem with the help of a diagram.
2. Explain the process of decomposition in detail.
3. Describe different types of ecological pyramids.
4. Explain productivity in an ecosystem (GPP and NPP).
5. What are ecosystem services? Explain their importance.
6. Describe the carbon cycle in nature.
7. Explain nutrient cycling with suitable examples.
8. Write a detailed note on structure and function of ecosystem.

🔹 Case Study Based Questions (New Pattern)

1. A student studied energy transfer between trophic levels and found loss of energy at each level.
   • (a) Name the law related to this observation
   • (b) Why does energy decrease at higher trophic levels?
2. In an aquatic ecosystem, pyramid of biomass is inverted.
   • (a) Name the ecosystem
   • (b) Why is pyramid inverted?

🔹 Assertion – Reason Questions (Repeated)

1. Assertion (A): Pyramid of energy is always upright.
   Reason (R): Energy is lost as heat at each trophic level.
2. Assertion (A): Decomposers are essential for ecosystem.
   Reason (R): They recycle nutrients back to the soil.
3. Assertion (A): Food web is more stable than food chain.
   Reason (R): It provides alternative food sources.

🔹 MCQs (Frequently Asked)

1. Which component forms the base of food chain?
   a) Consumers
   b) Producers ✅
   c) Decomposers
   d) Carnivores
2. NPP is equal to:
   a) GPP + R
   b) GPP – R ✅
   c) R – GPP
   d) None
3. Which pyramid is never inverted?
   a) Number
   b) Biomass
   c) Energy ✅
   d) None

🎯 Exam Tip for Students

✔ 5-mark questions usually come from:

• Energy flow
• Decomposition
• Ecological pyramids
• Productivity

✔ Always draw labelled diagrams for full marks

We are Completed Class 12 Biology Ecosystem Notes.

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